The invention relates to a binaural digital hearing aid system comprising two hearing aid units for arrangement in a user""s left and right ear, respectively, each of said units comprising input signal transducer means for conversion of a received input sound signal into an analog input signal, A/D conversion means for conversion of said analog input signal into a digital input signal, digital signal processing means for processing said digital input signal and generating a digital output signal, D/A conversion means for conversion of said digital output signal into an analog output signal and output signal transducer means for conversion of said analog output signal into an output sound signal perceivable to the user, a bidirectional communication link being provided between said units to connect a point in the signal path between the input signal transducer means and the digital signal processing means in one of said units with a corresponding point in the signal path between the input signal transducer means and the digital signal processing means of the other of said units.
For normally hearing persons the ability to localize sounds in space defined as binaural hearing ability is an important part of the sound perception. Typically the amplitude of sound received by the ipsilateral ear which is closer to the source of sound, is of greater amplitude than the sound received by the opposite contralateral ear. This difference in sound level, although often small by itself, is of great importance for a human being""s perception of the direction of an incident sound.
In the human hearing system binaural sound perception results from a complicated signal processing of sounds arriving at the left and right side ears, in which time/phase and frequency distribution of the sound plays a decisive role. Thus, time/phase differences and frequency enhancement are important for determining directions in the horizontal and vertical planes, respectively.
With conventional analog hearing aids persons suffering from a binaural hearing impairment, i.e. a hearing loss affecting both ears, the customary practice has been to use two separate hearing aids adjusted to compensate individually for the hearing loss of the respective ear for which the hearing aid is operative and compensation of the loss of binaural sound perception, although typically made even worse by the very use of a hearing aid in both ears, has in most cases by and large been ignored.
As a relatively simple compensation, it has been suggested for each of the two hearing aids of an analog system to use a microphone with a pronounced direction dependent characteristic to provide an analog signal the level of which changes, when the hearing aid is moved from a position pointing towards the sound source to other position with a minimum level, when the hearing aid points in a direction at right angles to the direction to the sound source.
In U.S. Pat. No. 3,509,289 a different concept for compensation of binaural hearing loss in an analog hearing aid system is disclosed involving the use of cross-coupled AGC circuitry for maintaining and enhancing the interaural level difference between contralateral and ipsilateral incident sound. In this system, the gain of each of a first and second amplifying channel is varied inversely with the output of the other channel by separate AGC circuits which are cross-coupled to stabilize the system.
With the introduction of digital signal processing in hearing aids a significant improvement of hearing aid performance has become possible and more advanced proposals for binaural hearing loss compensation have seen the light.
Thus, U.S. Pat. No. 5,479,522 discloses a hearing enhancement system comprising in addition to two hearing aid devices for arrangement in the left and right side ears, respectively, a body-worn pack comprising a remote digital signal processor connected to each of the hearing aid device by a down-link and an up-link for interactive digital processing of the audio signals for each ear based on signals received from both hearing aid devices. The common binaural digital signal processing is predetermined and limited to attenuation of noise and narrowing of the sound field or adapting the signal level in the two channels. The signals supplied to the common binaural signal processing are not affected by the individual hearing loss compensation in the two channels.
In addition, this prior art system reduces the comfort by requiring a separate body-worn signal processor in a addition to the two hearing aid devices and the physical links between the common binaural processor and the two hearing aid devices in the form of radio communications make the system susceptible to distortion affecting the quality of sound reproduction.
In WO 97/14268, a binaural digital hearing aid system is disclosed in which the need for a separate body-worn remote control processor has been eliminated by the use of two hearing aid devices for arrangement in the left and right side ears, respectively, each of which incorporates a digital signal processor to which not only the unprocessed audio signal generated by the microphone in the same hearing aid device is supplied, but also the unprocessed audio signal generated by the microphone in the opposite hearing aid device, the latter audio signal from each of the two devices being supplied to the respective opposite device through a bidirectional communication link.
This prior art system can be switched between distinct modes of either full binaural signal processing or performance as a conventional monaural hearing aid, which in one embodiment is done by giving the user the option of disabling the digital signal processor by either physically removing an external digital processing unit or by disabling a digital processor.
In the binaural processing mode of this prior art system no account is taken of the difference with respect to hearing loss and compensation between the two ears and, somewhat generalized, the system could be seen as an advanced digital substitute for the above-mentioned relatively simple binaural compensation using microphones with a pronounced direction dependent characteristic.
On this background, it is the object of the invention to provide an improved digital binaural hearing aid system in which the above-mentioned shortcomings of prior art systems have been eliminated to provide for a binaural signal processing, which for persons with a binaural hearing loss will restore binaural sound perception while taking into account the difference in hearing loss and compensation between the two ears.
According to the invention, a binaural digital hearing aid system as defined above is characterized in that the digital signal processing means of each unit is arranged to effect a substantially full digital signal processing including individual processing of signals from the input transducer means of the actual unit and simulated processing of signals from the input transducer means of the other unit as well as binaural signal processing of signals supplied, on one hand, internally from the input signal transducer means of the same unit and, on the other hand, via said communication link from the input signal transducer means of the other unit, said digital signal processing means including at least a first digital signal processor part for processing said internally supplied signal, a second digital signal processor part for processing the signal supplied via said communication link and a third digital signal processor part to effect common binaural digital signal processing of information derived from the signals processed in said first and second digital signal processor parts, said second digital signal processor part in each unit simulating the first digital signal processor part in the other unit with respect to adjustment parameters controlling the performance of said first signal processor part in said other unit.
Thereby, in the binaural hearing aid system of the invention each of the hearing aid units for the left and right side ears, respectively, performs in addition to digital signal processing adapted to compensate for the hearing loss of the ear served by the unit, a simulated full digital signal processing of sound signals received by the unit for the opposite ear and adapted to compensate for the specific hearing loss of that ear, as well as a common binaural signal processing taking into account both of the normally different compensation characteristics of both units.
By the advantageous embodiments and modifications of the system set out in the dependant claims the system can be designed for user operated switching between functioning as a binaural system and a conventional monaural hearing system, and the digital signal processing means in each hearing unit may be programmable to be switchable between different sound environments or listening situations by user operation, whereby programmed performance data for the first signal processing means of one unit is entered for programming of the second signal processing mens of the other unit, in which the simulated signal processing of signals supplied from the first unit is carried out.
By the provision of only a single bidirectional communication link between the two hearing aid units, the hearing aid system of the invention is less susceptible to signal distortion and interruption than the prior art systems described above.